Support gzipped message file

[David Bremner]

David Bremner <david at tethera.net> writes:

> This obsoletes the WIP series id:20190324033244.12909-2-david at tethera.net
>
> Compared to the WIP version, this has some general cleanup, and a
> switch to using an open file descriptor to create a stream rather than
> a FILE*.

I have made two minor changes  in git

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If anyone has any other suggestions, please let me know ASAP, otherwise
I may apply the Chomsky rule of code review.

d